Thiazine indigo compounds are an important series of pigments. Prior art processes for forming these pigments involve the reaction of certain o-aminomercapto- carbocycles or heterocycles with maleic acid or a derivative thereof in the presence of a solvent. The solvents useful for this purpose were recognized to be either a carboxylic acid (which also acted to catalyze the reaction) or an inert polar aprotic solvent.
Thus in DE 2 151 723 a process is described for making certain symmetrically substituted benzothiazine indigo compounds wherein certain substituted ortho-aminomercapto-carbocycles or heterocycles are reacted with a maleic acid derivative in a carboxylic acid, e.g. acetic acid. The pigments formed in this way are produced in good yield but they exhibit dull shades. By symmetrically substituted is meant that there is identical substitution on each of the thiazine rings.
In DE OS 25 36 120 a process is described for making certain other symmetrically substituted benzothiazine indigo compounds wherein certain substituted ortho-aminothiophenols are reacted with a maleic acid derivative in an inert polar aprotic solvent. The benzothiazine indigo compounds formed, allegedly display improved pigmentary properties, e.g. brighter and cleaner shade over the compounds made in a carboxylic acid solution, however, they can only be produced in poor yield.
There remains a need to provide further thiazine indigo pigments and an improved process of forming thiazine indigo pigments.